It is known that aromatic polyamide resins exhibit excellent thermal properties, chemical resistance, mechanical strength and electric insulating properties. Accordingly, fibers or filaments made from aromatic polyamides exhibit excellent thermal properties, for example, a high melting point and excellent heat resistance, and therefore, are useful as heat resistant fibers or filaments. Also, aromatic polyamide fibers or filaments exhibit excellent mechanical properties, for example, a high initial Young's modulus and an excellent tenacity, and therefore, are useful as rubber reinforcing materials, for example, tire cord, and as a reinforcing material for thermoplastic and thermosetting resin articles.
It is known that aromatic homopolymers or copolymers containing an aromatic component having two valence bonds oriented coaxially or in parallel to each other, for example, poly-p-phenylene terephthalamide, are useful for producing fibers or filaments having a high Young's modulus and tenacity. However, the above-mentioned type of aromatic polyamide, which is symmetrical in structure and has excellent mechanical strength, exhibits poor solubility in the usual solvents and has a poor shaping property.
The above-mentioned disadvantages in solubility and shaping property of the conventional aromatic polyamide can be somewhat reduced by introducing a recurring m-phenylene unit having two valence bonds oriented meta into a backbone chain in the molecule of the aromatic polyamide. However, the introduction of the m-phenylene unit causes a loss in the mechanical properties of the resultant aromatic polyamide article.
An aromatic copolyamide having high solubility can be obtained by introducing a recurring unit of the formula: ##STR4## wherein Y represents a member selected from the group consisting of ##STR5## into the backbone chain of the molecule of the conventional aromatic polyamide.
This type of aromatic copolyamide is disclosed in Japanese Patent Application Publication (Kokoku) No. 53-32838/1978, and is useful for producing fibers or films having excellent mechanical properties. However, the excellent mechanical properties can be created only by applying a drawing operation to the aromatic copolyamide fiber or film at a high draw ratio and an elevated temperature. This high draw ratio, high temperature drawing operation causes the production of the fiber or film to be expensive and poor in efficiency. Therefore, it is strongly desired to provide a new type of aromatic polyamide having not only excellent thermal properties and mechanical properties, but also, superior solubility in usual solvents, and being capable of readily shaped into various shaped articles, for example, fibers and films.